Hurricane Update: Did Global Warming Contribute to Hurricane Sandy’s Devastation?
- NYC 10/30/2012 by JUSTIN GILLIS (NY Times)

Was the bizarre storm called Sandy a product, in whole or in part, of human-induced climate change?

That may not be a top-of-mind issue for the millions of people who will spend coming weeks recovering from the damage. But it is an important scientific question, one whose answers could shed light not just on why this storm happened but also on what to expect in the future.

The first thing to say is that climate scientists are just not in a good position to answer it yet.

Some of them are already offering preliminary speculations, true, but a detailed understanding of the anatomy and causes of the storm will take months, at least. In past major climate events, like the Russian heat wave and Pakistani floods of 2010, thorough analysis has taken years — and still failed to produce unanimity about the causes.

But in interviews on Tuesday, several climate scientists made some initial points. A likely contributor to the intensity of Sandy, they said, was that surface temperatures in the western Atlantic Ocean were remarkably high just ahead of the storm — in places, about five degrees Fahrenheit higher than normal for this time of year. In fact, part of the ocean was warmer than it would normally be in September, when accumulated summer heat tends to peak.

Kevin E. Trenberth, a scientist with the National Center for Atmospheric Research in Boulder, Colo., said that natural variability very likely accounted for the bulk of that temperature extreme. And many of Sandy’s odd features derived from its origin as a “hybrid” storm — a merger of several weather systems, including a hurricane and a midlatitude storm that had earlier dumped snow in Colorado.

“My view is that a lot of this is chance,” Dr. Trenberth said. “It relates to weather, and the juxtaposition of weather systems. A hybrid storm is certainly one which is always in the cards and it’s one we’ve always worried about.”

But, he added, human-induced global warming has been raising the overall temperature of the surface ocean, by about one degree Fahrenheit since the 1970s. So global warming very likely contributed a notable fraction of the energy on which the storm thrived — perhaps as much as 10 percent, he said.

Other scientists are looking at this year’s historic loss of sea ice in the Arctic as a potential contributor to the track of Sandy, and possibly to the severity of the storm.

Summer sea ice in the Arctic has fallen by roughly half since the late 1970s, a change most climate scientists believe has been caused largely by human-induced warming. A large camp of experts, Dr. Trenberth among them, believe the weather effects have mostly been confined to the Arctic Ocean and surrounding land areas.

But some published research suggests the consequences extend much farther. The idea is that the loss of sea ice is altering the flow of the atmosphere enough to heighten the risk of severe weather in midlatitude regions like the United States.

In articles like this one, I have cited the work of Jennifer A. Francis, a Rutgers University climate scientist who is a leading proponent of this view. My colleague on the opinion side of The Times, Andrew Revkin, posted an analysis from Dr. Francis this week in which she noted that an atmospheric blocking pattern over Greenland — possibly linked, in her view, to the loss of sea ice in the nearby Arctic Ocean — had helped force the storm to make a left turn into the United States mainland.

“While it’s impossible to say how this scenario might have unfolded if sea-ice had been as extensive as it was in the 1980s, the situation at hand is completely consistent with what I’d expect to see happen more often as a result of unabated warming and especially the amplification of that warming in the Arctic,” Dr. Francis wrote.

Although Sandy began as a hurricane, drawing strength from evaporation at the warm ocean surface, scientists noted that by the end it was also pulling energy from a second source: the sharp differences in atmospheric temperature and pressure that normally drive winter storms.

Kerry A. Emanuel, a leading climate scientist at the Massachusetts Institute of Technology, pointed out that little analysis had been done of whether this type of storm might become more frequent or intense as the earth warms in the coming century.

“The only honest answer for hybrid events is that we don’t quite know what to expect with those,” Dr. Emanuel said. “My profession hasn’t done its homework.”

That is a sharp contrast with the state of knowledge involving hurricanes. Extensive computer modeling has suggested that the number of Atlantic hurricanes will stay the same or even decrease with global warming, but that the intensity of the storms that do occur is likely to increase. Dr. Emanuel predicted that Sandy would set off a scientific rush to perform the same kind of modeling for hybrid storms.

“I think there’s going to be a ton of papers that come out of this, but it’s going to take a couple of years,” he said.

Scientists don’t need fancy computer modeling to know that the biggest problem seen during Hurricane Sandy will become worse in the future: storm surge.

The ocean is rising relentlessly, and scientists say this is a direct consequence of global warming. Warm water expands, just as warm air does, and the warming of the ocean is one factor behind the rise. Another is that land ice the world over is starting to melt as the climate grows warmer, dumping extra water into the ocean.

Over all, the ocean rose about eight inches in the last century. The rate appears to have accelerated recently, to about a foot per century, and some scientists think it will accelerate further, so that the rise between now and the end of the century could exceed three feet. The problem will be exacerbated in places where land is also sinking, such as the mid-Atlantic region of the United States and southern Louisiana.

The likely effect, Dr. Emanuel said, is that coastal flooding on a scale that once happened only once or twice per century — the scale of Sandy, in other words — will become much more commonplace within the coming decades.

“There’s a reason why we build houses as far back from the beach and as high up as we do,” Dr. Emanuel said. “Sea-level rise is putting our built structures closer to the water line, in effect.”

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